Diarrhea, weight loss, and protein loss are common signs of intestinal disorders in horses. These signs can be caused by many different disorders, including infectious diseases, parasites, reactions to poisons or drugs, stress, changes in diet, and certain types of colic see Digestive Disorders of Horses: Specific Causes of Colic and their Treatment. Because the signs are similar for many intestinal disorders, determining the cause can sometimes be challenging and is often best left to a veterinarian. A correct diagnosis will lead to better treatment and, in many cases, a faster recovery.
Diarrhea in adult horses can be acute (sudden and often severe) or chronic (persisting a month or more). It is often difficult to diagnose the cause of chronic diarrhea. It can occur due to inflammatory or cancerous conditions involving the intestine or disruption of normal digestion. Possible causes include sand colic and abnormalities such as those associated with inflammatory bowel disease. The body's response to certain components of feed may play a role in chronic diarrhea of horses due to bowel inflammation, but has not frequently been established as a cause. Diarrhea in adult horses should be considered a serious event.
The causes of weight loss in horses are numerous and can involve many body systems. This discussion will cover only diseases of the digestive tract. Protein loss (called hypoproteinemia) may or may not be associated with weight loss. The disorders commonly associated with either of these signs are tumors, inflammatory bowel disease, and adverse reactions to treatment with nonsteroidal anti-inflammatory medication.
Salmonellosis is one of the most common infectious causes of diarrhea in adult horses. It is caused by many species of Salmonella bacteria and can cause severe disease if the bacteria or toxins it produces enter the bloodstream (septicemia). It can also cause inflammation of the small or large intestine (enteritis or colitis). Signs range from none to sudden, severe diarrhea, and even death.
The usual route of entry by the bacteria is by mouth. After entry, the organism multiplies in the intestines and causes intestinal inflammation. Young horses are the most susceptible. Penetration of bacteria into the intestinal surface contributes to tissue damage and diarrhea. Entry into the bloodstream may follow. The bacteria can also live in the lymph nodes and can cause infection in other parts of the body, such as the liver, lungs, joints, or bones, especially in foals.
Some horses that show no signs of disease may be carriers of Salmonella. Although most horses clear the infection in a few weeks, some can carry the bacteria for prolonged periods and shed it in the manure when stressed. In adults, most cases of disease develop after the stress of surgery or anesthesia, alteration in diet, treatment with antibiotics, or transport. Mares may shed the bacteria while giving birth and infect the newborn foal. Salmonellosis in horses hospitalized for other causes is a major problem for equine clinics because horses that are hospitalized experience one or more of the stressors listed above. In these circumstances, a large number of vulnerable horses are present and carriers may shed the bacteria.
In newborn foals, the bacteria or its toxins are more likely to spread beyond the intestine into the bloodstream. Illness can develop suddenly and be severe; signs generally include lethargy, reduced suckling, and a high fever. Death occurs in 24 to 48 hours. In some foals these sudden signs of illness are not noticed. If the bacteria spread to joints or bones, the foal can become lame and have a fever.
In adult horses, intestinal inflammation is the most common form of the disease. Initially, there is fever, followed by severe, watery diarrhea. Straining during defecation can occur. The feces may have a foul odor and contain mucus, shreds of mucous membrane, and in some cases, blood. Abdominal pain and colic can also occur. Affected horses are severely dehydrated and many die within 24 hours of the onset of diarrhea if not treated promptly. A milder illness may develop in some adult horses. The signs include mild fever, soft feces, poor appetite, and dehydration.
A diagnosis of salmonellosis is based on signs and identification of the organism after laboratory examination of feces and tissues from affected animals.
Early treatment is essential for cases of salmonellosis in which the horse or foal has signs of dehydration, pain, or bacteria in the bloodstream (septicemia). Broad-spectrum antibiotics are given intravenously to treat septicemia. Oral medication may be provided in drinking water because affected horses are thirsty due to dehydration, and their appetite is generally poor. Some horses with severe or ongoing fluid and electrolyte (salt) loss will require treatment with intravenous fluids.
There is controversy regarding the use of antibiotics for intestinal salmonellosis (the type seen more commonly in adults). Oral antibiotics may damage the normal population of intestinal bacteria, which aid in digestion and help protect the animal from other, disease-causing bacteria. There is also concern that antibiotic use may increase the number of bacteria that can resist antibiotics and make the infection more difficult to treat. Horses with acute intestinal salmonellosis will probably be given fluids containing electrolytes (salts) intravenously to correct deficiencies of sodium and potassium. Plasma with a high concentration of antibodies against Salmonella may also be given. Some horses may require treatment with medications given via a tube that is passed into the stomach. These medications are designed to help heal the intestine and absorb toxins produced by the bacteria.
Although the signs of disease may disappear, eliminating the bacteria from the body is difficult, particularly in adult horses.
Control and prevention are focused on reducing the likelihood of exposure to the bacteria and promoting the overall health of the horse. A horse could be exposed to the bacteria by contacting the feces of an animal (for example, another horse or a bird or rodent) that is shedding the bacteria. It is advisable to make every effort to avoid introduction of a carrier animal that is shedding the bacteria into the environment. Horses should be purchased from farms known to be free of the disease and should be housed away from the resident horses for about 2 to 3 weeks while their health status is monitored. Feed and water supplies should be high quality and must be protected from fecal contamination by rodents, birds, or any other animal that may be shedding the bacteria in its feces. Should the bacteria be detected in the environment, any contaminated living areas should be thoroughly cleaned and disinfected. It is critical that a well-thought-out plan for cleaning be made and all label instructions on disinfecting products be followed. Because Salmonella can also infect humans, anyone working with infected horses or wastes from infected horses should be aware of the risks and the need for good personal hygiene.
Potomac Horse Fever
Potomac horse fever is a syndrome producing mild colic, fever, and diarrhea in horses of all ages, as well as loss of foals in pregnant mares. It is caused by Neorickettsia risticii bacteria. The bacteria are found in parasites called flukes, which have been isolated from freshwater snails and appear to be present in a number of insects. One route of exposure is believed to be the horse's accidental ingestion of aquatic insects containing infected flukes. The disease is seen in spring, summer, and early fall and is associated with pastures bordering creeks or rivers. It has recently been noticed that having lights on in barns at night attracts insects, which may fall in feed or water buckets.
Infection of the small and large intestine results in inflammation of the large intestine (colitis), which is one of the principal signs of the disorder. Early signs of Potomac horse fever include mild depression and loss of appetite, followed by a mild to high fever. At this stage, a veterinarian may be able to detect decreased intestinal sounds. A definitive diagnosis is based on identifying the Neorickettsia risticii bacteria in the blood or feces of infected horses using a DNA test.
Within 24 to 48 hours, a moderate to severe diarrhea and abdominal discomfort develops in about 60% of affected horses. Some horses develop severe blood poisoning and dehydration. Founder or laminitis see Bone, Joint, and Muscle Disorders in Horses: Laminitis (Founder) can occur as a severe complication of the disorder in up to 40% of affected horses.
Several months following disease in pregnant mares, miscarriage of the foal due to fetal infection with Neorickettsia risticii may occur. The miscarriage is accompanied by an enlarged and retained placenta.
Sick horses are not contagious and can be housed with other horses. Potomac horse fever can be treated successfully with an appropriate antibiotic, if it is given soon after the disease begins. A response to treatment is usually seen within 12 hours. This is associated with relief from fever, followed by an improvement in attitude, appetite, and bowel sounds. If treatment is started early, signs frequently resolve by the third day of treatment. Generally, antibiotics are given for no more than 5 days. In horses with signs of enterocolitis, fluids and a nonsteroidal anti-inflammatory drug will likely be prescribed. Founder, if it develops, is usually severe and often resistant to treatment.
Several vaccines are commercially available; however, they do not appear to be very effective. Reducing the number of snails in rivers and ditches may be attempted to lessen sources of infection. No risk to humans from this disease is known.
Clostridia-associated Intestinal Inflammation
Clostridium bacteria have been diagnosed as a cause of intestinal inflammation in horses and foals. The exact role of the bacteria is still unclear, and it may be that multiple factors—such as stress, antibiotic use, and altered diet—are involved in the development of this disorder. Disease due to this infection is more common in foals but appears to occur in adult horses as well.
Signs of this condition include diarrhea with or without blood, colic, poor appetite, lethargy, and sudden death. These signs are similar to those seen in other causes of intestinal inflammation. Affected foals (usually less than 3 days old) often have bloody diarrhea and colic. Several foals on the same farm may be affected, but usually only one case occurs on a farm at a time. Diagnosis is based on signs and identification of the bacteria or the toxins produced by the bacteria in feces or tissue samples from the infected horse or foal. If you are asked to bring in a fecal sample for testing, follow your veterinarian's instructions in regard to the handling of these samples to preserve the bacteria.
In humans, Clostridium bacteria have been identified as an infection common in hospitals. This may also be seen in horses receiving hospital care. Veterinary hospital personnel are aware of this possibility and take special precautions to prevent the spread of the disease.
Treatment is similar to that for other causes of intestinal inflammation but is not always effective if the newborn foal has severe disease due to the infection. Treatment includes intravenous fluids and prescription drugs. Newborn foals may be given medication to help prevent stomach ulcers. Broad-spectrum antibiotics may also be prescribed. Foals with colic associated with ingestion of milk often require intravenous fluids and nutritional support. Drugs that are given via a tube into the stomach may help absorb the toxins produced by the bacteria and help the intestine heal. Intensive treatment may be required for several days until the condition improves.
Colitis-X is an extremely rapid, usually fatal disease of horses, with a sudden onset of profuse, watery diarrhea and development of shock. Many affected horses have a history of stress. The cause of colitis-X is unknown, although multiple factors have been proposed, including severe salmonellosis, infection with clostridia, and endotoxemia (absorption of bacterial toxins into the bloodstream from the gut). Disease onset is often closely associated with stress such as surgery, anesthesia, strenuous athletic events, or transport.
The disease may begin with a short period of fever, but the horse's temperature soon returns to normal or below normal. Rapid breathing, rapid heart rate, and lethargy are present. Severe diarrhea develops, followed by extreme dehydration and shock. Death may occur within 3 hours of onset of signs. In some horses the progression of the disease is so rapid that the only sign is sudden death. In less sudden cases, death occurs within 24 to 48 hours. Colitis-X is almost always fatal.
Treatment for colitis-X usually is not effective but is similar to that for salmonellosis see Digestive Disorders of Horses: Salmonellosis. Large volumes of intravenous fluids are needed to treat the severe dehydration, and electrolyte (salt) replacement is often necessary.
A number of parasites are known to cause diarrhea in horses. Both large and small worms called strongyles see Digestive Disorders of Horses: Large Strongyles have been linked to chronic diarrhea in horses and foals. Infections with Giardiasee Digestive Disorders of Horses: Giardiasis and Cryptosporidiumsee Digestive Disorders of Horses: Cryptosporidiosis protozoa can cause diarrhea in horses and foals.
Intestinal Disease Caused by Eating Sand or Dirt
Consumption of large amounts of sand, which then accumulates in the large intestine, can produce diarrhea, weight loss, or colic. Sand may be eaten accidentally along with food when the horse or foal is kept on sandy pasture or is fed hay or grain in a sandy area (paddock, stall, or pasture). Some horses or foals develop a habit of eating dirt and sand if it is in their environment.
A veterinarian can diagnose this condition based on history of exposure to a sandy environment, the presence of sand in the feces, “sand sounds” in the abdomen, and abdominal x-rays or ultrasonography. Treatment involves administering a fiber product (usually psyllium seed hull) by a tube inserted up through the nostril and into the stomach or added to the concentrate feed daily. Diarrhea generally resolves within 2 to 3 days after the start of treatment; however, 3 to 4 weeks of treatment is often necessary to remove all of the sand. The treatment may need to be repeated if the horse or foal is not removed from the source of sand. Preventive treatment with a fiber product may be recommended in areas where this condition is common.
Other Causes of Diarrhea in Horses
Some horses develop soft feces when first introduced to lush pastures, alfalfa hay, or a temporarily stressful situation such as a trailer ride, racing, showing, or visit to a veterinary hospital. This change in fecal consistency is not of medical significance as long as the horse is healthy in all other regards. It is important that horses with diarrhea have a physical examination and appropriate laboratory tests to exclude infectious causes and to determine whether treatment is required. Usually, the fecal consistency returns to normal when the horse adapts to its new diet or the stressful situation resolves.
Other causes of diarrhea or semiformed to watery feces in horses include grain overload, partial failure of the heart resulting in thickening of the intestinal wall with retained fluid, peritonitis, antibiotic treatment, kidney or liver failure, and numerous poisons (such as certain chemicals, plants, and insects). Diarrhea can also be seen while an impaction of the large intestine is resolving or being treated.
Diarrhea in Foals
Diarrhea in foals may be caused by bacterial or viral infection, parasites, and nutritional or environmental changes.
Foal Heat Diarrhea
At about 4 to 14 days after birth, foals often develop a mild diarrhea that resolves on its own. Although the cause is unknown, it may be associated with changes in the normal bacteria in the foal's intestines or alteration in diet as the foal begins to eat small amounts of hay and grain. The condition is often referred to as foal heat diarrhea because it appears at about the same time as the dam is undergoing her first estrous cycle after the foal's birth.
The foal remains active and alert and has a normal appetite. Vital signs remain normal. Feces are semiformed to watery and not foul-smelling or passed more frequently than is normal. Monitoring is important to ensure the foal's condition does not worsen. Specific treatment is usually not necessary, but application of a protectant to the skin around the buttocks helps prevent irritation from the diarrhea.
Several bacterial infections can cause intestinal inflammation and blood infection in newborn and young foals. Organisms that can be involved in diarrhea include Salmonella and Clostridium species and others. Intensive antibiotic treatment, correction of fluid loss and electrolyte (salt) abnormalities, and nursing care are usually needed. If a veterinarian determines that transfer of antibodies from the mother during suckling was not sufficient, a blood plasma transfer into the foal's blood may be performed. Your veterinarian will prescribe medication based on the diagnosis, the condition of the foal, and other factors.
Intestinal infection with the bacteria Lawsonia intracellularis has been associated with outbreaks of diarrhea, rapid weight loss, colic, accumulation of fluid under the skin (edema), and lowered levels of protein (due to loss from the intestines) in weanling foals on breeding farms in Canada and the United States. Because it is difficult to diagnose this condition, some veterinarians will initiate treatment for this bacterium in foals when other causes of diarrhea have been excluded and there is evidence of exposure. Treatment with appropriate antibiotics has been successful, and a response to this treatment is considered confirmation of the diagnosis.
Viral Diarrhea in Foals
Rotavirus is the main cause of viral diarrhea in foals, but other viruses such as coronavirus may also be responsible. Signs of rotaviral infection can include lethargy, colic, loss of appetite, and profuse, watery, foul-smelling diarrhea. It is usually seen in foals less than 2 months old. Younger foals typically have more severe signs. The diarrhea usually lasts 4 to 7 days, although it can persist for weeks. Rotavirus destroys cells in the lining of the small intestine, causing poor absorption of nutrients. Lactase, an enzyme required for digestion of milk, becomes deficient. When this occurs, undigested lactose passing into the large intestine causes diarrhea. Treatment is generally supportive.
This type of diarrhea is highly contagious. In situations where multiple foals are housed on the same farm, sick foals should be isolated in the stall or barn in which the foal originally became ill or moved to a special isolation facility. Strict hygiene and disinfection practices should be followed, including use of disposable gloves and foot covers, handwashing, and disinfection of stalls and equipment with compounds that meet Environmental Protection Agency standards. Because stalls with dirt floors are difficult to adequately clean and disinfect, removal of the top layers of dirt may be required. Fecal material of sick foals removed from stalls should not be spread on pastures that are used for horses and foals. Care should be taken with all equipment that may contact the manure of ill foals so that it is either thoroughly cleaned and disinfected prior to use with healthy foals or is used only for care of ill foals. For example, a wheelbarrow and pitchfork used for cleaning stalls of ill foals should never be used to feed foals and should never be used to clean the stalls of healthy foals unless it is thoroughly cleaned and disinfected.
A vaccine is available for pregnant mares to help protect their foals. The vaccine causes the mares to produce antibodies to rotavirus. These protective antibodies are passed to their foals during suckling of the first milk (colostrum) from the mare.
Miscellaneous Causes of Diarrhea in Foals
Nutritional diarrhea can result from overfeeding (for example, when a foal is reunited with the mare after a period of separation) and improper nutrition (as when orphan foals are being fed calf milk, a replacer formula, or sucrose). Lactose intolerance in foals is rare and can be determined by a lactose tolerance challenge test. Diarrhea can also develop when foals consume indigestible substances such as roughage, sand, dirt, or rocks.
Diarrhea in foals has been reported to be associated with infection by the parasites Strongyloides westeri, Parascaris equorum, and Cryptosporidium species see Digestive Disorders of Horses: Cryptosporidiosis.
Several types of tumors can affect the digestive tract, with the primary sign being chronic weight loss. However, gastrointestinal tumors are rare, so veterinarians often investigate other more common causes of weight loss first. Diagnosis is usually made by excluding other possible causes of weight loss and by examining tissue collected during exploratory surgery or initial treatment. Some cancerous conditions can be diagnosed using endoscopy, ultrasound, rectal examination, or microscopic examination of abdominal fluid. Treatment of cancerous gastrointestinal tumors in horses is generally not attempted because the outlook for longterm recovery is poor. However, a few cases of gastrointestinal cancer have been treated, which improved the well being of the horses at least temporarily.
Inflammatory Bowel Disease
This disease occurs when inflammatory cells accumulate in the small and large intestine and regional lymph nodes. The inflammation may be limited to only a short segment of the bowel or it may be more widespread. This condition interferes with absorption of nutrients and causes a loss of protein from the blood. Diarrhea may or may not be present. Diagnosis is based on signs, physical examination, tests for low blood protein or malabsorption, and intestinal or rectal biopsy. Your veterinarian may detect thickened intestines or enlarged abdominal lymph nodes based on rectal examination. The cause of this disease is not well understood. An altered immune response to a common intestinal exposure (such as feed, parasites, or bacteria) has been suggested.
Medical treatments that may reduce the inflammatory response in the intestine have been tried with limited success. Supportive nutritional care is often prescribed. The usual program involves the frequent feeding of good-quality, high-energy feeds. The longterm outlook is frequently poor. If only a limited and accessible section of the bowel is affected, your veterinarian may recommend surgery to remove the affected portion of the intestine.
Nonsteroidal Anti-inflammatory Drug Toxicity
If the drugs phenylbutazone, flunixin meglumine, or other nonsteroidal anti-inflammatory agents are administered at high doses or for prolonged periods, they can cause protein to be lost from the blood into the intestines. Signs include mouth sores, loss of appetite, lethargy, weight loss, diarrhea, and colic. Stomach and intestinal ulcers can also result from this condition. Your veterinarian can make a tentative diagnosis based on the history of drug administration, signs consistent with this condition, and the presence of reduced blood protein. Stomach ulcers can be confirmed using an endoscope inserted through the mouth or nasal passages and then into the stomach. Treatment includes discontinuing the use of phenylbutazone or any other nonsteroidal anti-inflammatory drug. Reducing the production of stomach acid with medications may be beneficial. Changes in diet can help some horses. Surgery may be required if scarring of the intestines has resulted in partial obstruction.
Malabsorption and Maldigestion
Sometimes horses have a defect in the ability of the digestive tract to incorporate nutrients into the body. There are 2 main causes: malabsorption and maldigestion.
Malabsorption is the failure of nutrients to pass from the inside of the intestines to the bloodstream. Many diseases can cause a malabsorption syndrome by altering the normal function of the small intestine. Protein loss may coexist with this condition and prove more harmful than the malabsorption itself. Maldigestion is usually caused by a missing or insufficient enzyme; it is the inability to digest certain foods within the intestines. In horses, diseases of malabsorption are much more common than are diseases of maldigestion. Some diseases involve both maldigestion and malabsorption, such as is seen in foals with lactase deficiency, an inability to digest the sugar in milk see Digestive Disorders of Horses: Foals with Lactase Deficiency (Lactose Intolerance).
Signs of malabsorption and maldigestion vary, depending on the underlying disease condition. Insufficient energy, weight loss, and possibly low blood protein concentrations are characteristics of these syndromes. Longterm weight loss, muscle wasting, or a reduced growth rate are common. Excessive eating may be seen, because the nutrients that are taken in do not effectively stimulate the brain areas that normally indicate fullness. More commonly with small-intestinal malabsorption, loss of appetite is present. Diarrhea may occur; however, small-intestinal disease may be extensive before diarrhea develops because the large intestine can compensate and absorb the increased fluid. In adult horses, diarrhea usually indicates large-intestinal disease.
Abnormal pain may result from bowel inflammation, abscesses, adhesions, or partial obstruction. Abdominal fluid buildup (ascites) and weakness may develop later in the disease, especially if protein loss is present. Skin and eye abnormalities, arthritis, hepatitis, and kidney disease may indicate immune system reactions, particularly with inflammatory bowel disease. Skin abnormalities seen with malabsorption-related skin disease include a thin hair coat, patchy hair loss, and areas of scaling and crusting that are often symmetrical.
Small-intestinal malabsorption cannot be determined by clinical examination or by routine laboratory data. The veterinarian must exclude more common causes of weight loss before a diagnosis can be made. Determination of the primary underlying disease process is also necessary to establish an appropriate treatment regimen and outlook.
The causes of the disease leading to malabsorption must be determined before treatment can be started. Specific treatment for most causes is not available; however, if the cause is parasite damage, it can sometimes be corrected with antiparasitic medications. Anti-inflammatory agents (such as nonsteroidal anti-inflammatory drugs or corticosteroids) may help decrease inflammation within the intestine, although care must be taken in their use. Follow your veterinarian's instructions about dosages and administration times carefully.
Horses that will not eat may have to be force-fed through a stomach tube. Intravenous feeding may be necessary for horses that refuse to eat or for those that cannot tolerate force-feeding. However, intravenous nutrition is difficult to continue on a longterm basis. For most adult horses with conditions causing malabsorption or maldigestion, the treatment is often unsuccessful and the outlook for recovery is poor.
Malabsorption and weight loss sometimes occur after viral enteritis, because the infection destroys the villi in the intestine. The villi are hair-like projections of the inner surface of the intestine that increase the area for uptake of nutrients. The loss of the villi means a reduction in the nutrients absorbed from food. Replacement of the villi may take weeks to months in severe cases.
Foals with Lactase Deficiency (Lactose Intolerance)
Lactose deficiency may be congenital but is more commonly acquired and temporary in foals. Foals with lactase deficiency commonly have diarrhea, poor growth rate, and an unthrifty appearance. Signs include flatulence, mild abdominal discomfort, or bloating after intake of milk.
Foals with temporary lactase deficiency caused by diarrhea often respond well to feeding of lactase-treated milk until the small-intestinal lining has regenerated. Foals that can tolerate it should be fed small amounts of high-quality roughage or grain to help meet their energy needs. Young foals may benefit from intestinal rest (withdrawal of milk feeding) while the intestinal lining heals. Dietary change to a soy-based, non-lactose-containing milk replacer and early weaning may be necessary for foals with lactose deficiency that does not respond to other treatments.
Last full review/revision July 2011 by Peter D. Constable, BVSc (Hons), MS, PhD, DACVIM; Gordon J. Baker, BVSc, PhD, MRCVS, DACVS; Joseph A. DiPietro, DVM, MS; Walter Ingwersen, DVM, DVSc, DACVIM; John E. Madigan, DVM, MS; James N. Moore, DVM, PhD; Michael J. Murray, DVM, MS; Sofie Muylle, DVM, PhD; Stanley I. Rubin, DVM, MS, DACVIM; Susan D. Semrad, VMD, PhD, DACVIM; Josie L. Traub-Dargatz, DVM, MS, DACVIM